The most conventional date timepieces are described in particular in the work entitled “Les montres compliquées” (A Guide to Complicated Watches) by Francois Lecoultre and edited by Editions Horlogères in Bienne.
In an instantaneous date mechanism, star-wheels for the days of the week, the month of the year and the date are all actuated by the same perpetual lever, which is pivotally mounted relative to the plate or to a bridge of the timepiece movement, and which accomplishes its date change function when the day changes, in an abrupt motion, at a precise moment, by the action of a beak and two clicks, comprised in said perpetual lever. This jump is performed around midnight.
This perpetual lever is returned by a spring to a rest position where one of its beaks abuts on a first sector of a month cam, carried by a month star-wheel with 12 teeth which completes one revolution per year, or a star-wheel with forty-eight teeth completing one revolution in four years. The radius of this month cam is representative of the number of days in the month concerned, either in the form of a notch of greater or lesser depth, or in the form of a portion that protrudes to a greater or lesser extent.
This perpetual lever is made to pivot abruptly by a wheel, which is connected to the movement and completes one revolution in 24 hours, carrying a pin for driving a heart-piece against a roller mounted on a lever returned by a spring. When the heart-piece passes a tip, this triggers the abrupt motion of a finger which drives a beak of the perpetual lever.
During its pivoting motion, the perpetual lever moves away from the month cam, and returns to abut thereon at the end of its motion, either in the same position, if pivoting occurs during the month, or to abut on the sector next to the first sector of the month cam, if the latter pivoted when actuated at the time of a change of date.
Another beak of the perpetual lever controls the pivoting of a day-of-the-week star-wheel. Since the sequence of days is perpetual, no particular mechanism is required, since it is sufficient to increment the position of this star wheel by one step.
The pivoting of a thirty-one star-wheel with thirty-one teeth is achieved via a first thirty-one click, which is pivotally mounted on the perpetual lever and whose pivoting is limited by a pin fixed to said lever. This thirty-one star-wheel pivots integrally with a first cam and a second cam both of which are snails.
The first snail cam is arranged to cooperate with a first feeler-spindle comprised in a pivoting month lever, returned towards said first cam by a spring. The function of this first cam is to cause the month lever to drop at the appropriate time. The month lever includes for this purpose, opposite the feeler-spindle, a second feeler-spindle arranged to cooperate with the month star-wheel with 12 teeth, and to push said month star-wheel by one tooth when the feeler-spindle drops from the large radius to the small radius of the snail.
The second snail cam includes a single notch, against which a second click acts, called the month-end adjustment click, carried by the perpetual lever, and rests thereon via a spring, and this second click only functions for months of less than 31 days.
At the end of a February with 28 days, the beak of the perpetual lever is in the deepest notch of the month cam, and, during the change from the 27th to the 28th of the month, the second click engages in the notch of the second snail cam. This particular setting of the perpetual lever allows the second click to drive four teeth of the thirty-one star wheel together, when the perpetual lever jumps around midnight on the 28th of the month. Simultaneously, the first snail cam actuates the pivoting of the month lever, which drives the month star wheel, and thus the month cam, to pass to the sector for the next month, which in this case is March. Consequently, the perpetual lever takes a new rest position during the month of March, which is such that the drop of the second click is delayed by four days. Thus, from the 28th March to the 1st April, this second click only drives one tooth of the thirty-one star wheel at a time.
Likewise, the notch of the month cam for the months of thirty days is arranged so that the second click drives two teeth of the thirty-one star wheel on the evening of the 30th of the month.
Likewise, the notch of the month cam for the months of February with 29 days in leap years is arranged so that the second click drives three teeth of the thirty-one star wheel on the evening of the 29th of the month.
This tried and tested system requires two clicks to ensure the proper operation of the instantaneous perpetual date mechanism.
CH Patent Application No. 660440A3 in the name of DUBOIS & DEPRAZ SA discloses a perpetual calendar mechanism which includes a large lever driven by the movement and which includes five fingers and one click. This click abuts on the periphery of a snail cam fixed to a thirty-one day wheel. Said wheel carries an actuating finger able to cooperate with a forty-eight month wheel. A first finger, formed by one end of the lever, is located on the path of an actuating finger, which can be driven by a pin carried by the twenty-four hour wheel. This actuating finger carries a convex dorsal part forming a cam, which actuates said perpetual date mechanism. In leap years, a finger enters into contact with a lever comprised in a leap year cam. A third finger cooperates with a twelve month cam. A fourth finger forming a drive beak cooperates with the toothing of a thirty-one day wheel carrying the snail cam, said fourth finger works every day, whereas the click carried by the lever only works at the ends of the months. A fifth finger forming a second drive beak cooperates with a seven day star wheel. Due to this construction, disruptions are different depending on whether they occur for days with one jump (from 1st to 27th) or days with several jumps (28th to 31st). Consequently the torque is used irregularly and the behaviour of the mechanism differs according to the length of the month in progress.
EP Patent Application No 2503411 in the name of MONTRES BREGUET SA describes a calendar mechanism for a timepiece, which includes a movement arranged to control, once a day, the pivoting motion of a perpetual lever comprised in said calendar mechanism, said mechanism comprising means for of driving a perpetual twelve cam, which includes twelve sectors for the successive months, of different radial dimensions according to the duration of each month, and which completes one revolution per year, characterized in that said mechanism has a single click and includes a single click finger, hinged to said perpetual lever and arranged to cooperate directly with a toothing comprised in a thirty-one ratchet wheel which pivots integrally about a pivot axis of a thirty-one snail cam directly or indirectly controlling a calendar display means, and a second cam determining the position of a countdown mechanism arranged to adjust the duration of cooperation between said click finger and said wheel according to the current month and the current day of the month and to determine each day the number of teeth of said wheel to be actuated, and said countdown mechanism measuring the duration of the current month on said perpetual twelve cam and according to the position of a pin comprised therein, allowing, limiting or preventing the cooperation between a beak of said click finger and said thirty-one ratchet wheel.
EP Patent Application No 1349020 A1 in the name of ROGER DUBUIS discloses a calendar timepiece with a large format display and instantaneous jump mechanism, including a time indicator train, and calendar wheel set including a wheel with thirty-one teeth, a units wheel with thirty teeth plus a space corresponding to one tooth for driving a units pinion with ten teeth and a wheel with four teeth for driving a tens star-wheel with four teeth, an annual cam integral with a wheel with twelve teeth, and drive means connected to the time indicator train for driving the calendar wheel set by one revolution per month and the annular cam by one revolution per year, in which the calendar wheel set is integral with a correction element, and the drive means include an instantaneous jump cam integral with a wheel connected to the time indicator train to complete one revolution per day, a drive lever equipped with a retractable drive finger, elastic means pressing said drive lever against the instantaneous jump cam, a correction lever including a retractable drive finger in mesh with said correction element, a feeler-spindle intended to detect the position of said annual cam, and elastic means for connecting these levers to each other, to place the retractable drive finger of the correction lever selectively in mesh with the correction element as a function of the annual cam position detected by the feeler.
EP Patent Application No EP1524564 A1 in the name of ROTH & GENTA describes a timepiece including an annual or perpetual calendar display mechanism, including at least one month cam wherein the mechanism further includes a movable element for indicating the number of days in the month, which can be moved with respect to at least one fixed indicator element and a connection for connecting said movable indicator element to the month cam, so that, during each month, this movable element for indicating the number of days in the month occupies, with respect to the fixed indicator element, a position characteristic of that of the month cam.